/Engine/Source/Runtime/Engine/Public/Slate/SGameLayerManager.h
Function will instruct internal DPI computations to use a provided reference viewport size instead of the actual viewport size. After the DPI will be retrieved it will be scaled down with the ratio between the actual viewport size and the provided one. Check GetGameViewportDPIScale() for more information.
void SetUseFixedDPIValue ( const bool bUseFixedDPI, const FIntPoint RefViewportSize )